The purpose of this research proposal is to elucidate the role of the basic Helix Loop Helix (bHLH) proteins XNGR1 and Neuro-D in neural development. bHLH proteins are transcription factors which have been implicated in cellular determination and differentiation during hematopoiesis, myogenesis and neurogenesis. In the Xenopus CNS, XNGR1 has been proposed to act as a proneural gene and NeuroD as a differentiation factor however, little is know about their direct effect on the steps of neural development. The sequential expression of these transcription factors suggests that a cascade of neurogenic steps occurs during neural development and that specific events along this cascade are directed by XNGR1 and Neuro-D. To test this hypothesis, the initial expression of XNGR1 and Neuro-D will be correlated to specific developmental events during neurogenesis. In addition, XNGR1 and Neuro-D will be misexpressed in Xenopus embryos to determine which events in neural development are under the direct control of these transcription factors. Expression of the myogenic bHLH transcription factor MyoD results in the cessation of cell cycle by increasing levels of the p21 cyclin inhibitor. A Xenopus p21 homologue, p28, has been identified and isolated to neurogenic regions of the Xenopus CNS. To test the hypothesis that p28 links neuronal differentiation to cessation of the cell cycle, XNGR1 and NeuroD will be ectopically expressed and alterations on p28 expression ascertained. These studies will define the function of XNGR1 and Neuro-D in the control of specific steps along a neurogenic cascade during normal vertebrate neuronal development.